1. Field of the Invention
The present invention relates to a superconducting wire having a stacked structure comprising an electrically conductive substrate, an oxide superconductor, and an electrically conductive material which is substantially not reactive with said oxide superconductor wherein said electrically conductive material is impregnated in said oxide superconductor such that said electrically conductive material fills gaps among crystal grains in said oxide superconductor while reaching the interface between said substrate and said oxide superconductor, which is suitable for use in various superconducting magnets and coils. The present invention also relates to a process for the production of said superconducting wire.
2. Related Background Art
There are known so-called Y-series oxide superconductors and so-called Bi-series oxide superconductors which exhibit a superconductivity at a temperature which is higher than the boiling point of liquid nitrogen. As for the production of a wire of any of these superconductors, there are know the following two manners: manner 1 comprising charging a metal tube with a superconducting material or a raw material capable of providing a superconducting material and subjecting the resultant to wire drawing, wherein if necessary, heat treatment is conducted before or after the wire drawing processing, to obtain a wire; and manner 2 comprising forming a film comprising an oxide superconductor on a tape-like shaped substrate by way of a thin film-forming process such as a sputtering process to obtain a wire.
However, as for these oxide superconductors, there are problems such that they are inferior in workability, i.e., they cannot be easily processed as metals; and in the case of subjecting them to wire drawing by means of rolling or dies, gaps are liable to occur among their crystal grains because the crystal grains are difficult to be deformed.
In addition, in the case of producing of a wire of such oxide superconductor in accordance the foregoing manner 1, problems are liable to entail in that when the metal tube is not charged with the oxide superconductor in such a state that the inside of the metal tube is entirely and uniformly filled with the oxide superconductor, the resulting wire becomes such that is practically not usable, and in addition, upon cooling the metal tube having the oxide superconductor contained therein, a reduction is caused in the contact of the oxide superconductor with the metal tube, wherein the resulting wire eventually becomes to be not usable in practice.
In order to eliminate the foregoing problems in the prior art, Japanese Unexamined Patent Publication No. 37623/1990 (hereinafter referred to as xe2x80x9cdocument 1xe2x80x9d) proposes a manner of charging an aluminum tube with an oxide superconductor to obtain a composite body comprising the aluminum tube charged with the oxide superconductor, and subjecting the composite body to heat treatment at a temperature of 900 to 1000xc2x0 C. wherein the aluminum of the composite body is molten away to expose the oxide superconductor which is followed by being sintered. In document 1, there is described that according to this manner, the oxygen content of the oxide superconductor can be controlled as desired and the occurrence of gaps among the crystal grains of the oxide superconductor can be prevented.
Similarly, Japanese Unexamined Patent Publication No. 276516/1989 (hereinafter referred to as xe2x80x9cdocument 2xe2x80x9d) proposes a manner of inserting a compact of an oxide superconductor in a silver tube and charging silver powder in the silver tube such that a clearance present between the oxide superconductor and the silver tube is filled with the silver powder. In document 2, there is described that according to this manner, an improvement can be attained in the contact of the oxide superconductor with the silver tube.
Further, Japanese Unexamined Patent Publication No. 241826/1988 (hereinafter referred to as xe2x80x9cdocument 3xe2x80x9d) proposes a manner providing a wire-drawn substrate having a surface comprised of a copper material or copper alloy material, forming a thin film composed of a superconducting material on the surface of the substrate by means of a thin film-forming process and subjecting the resultant to heat treatment. In document 3, there is described that being somewhat different depending upon the kind of the superconducting material, the heat treatment is usually conducted at a temperature of 800 to 1000xc2x0 C. for a period of time of 1 to 100 hours.
However, any of the manners described in documents 1 to 3 has still problems required to be solved. Particularly, in the case of the manner disclosed in document 1, because aluminum is of a melting point of about 660xc2x0 C., when the composite body comprising the aluminum tube charged with the oxide superconductor is heat-treatet at a temperature of 900 to 1000xc2x0 C., it is extremely likely that the aluminum of the composite body is oxidized with the oxygen contained in the oxide superconductor before it is removed from the oxide superconductor. Particularly, in the method described in document 1, there are aluminum materials embedded in the recesses or grain boundaries present at the surface of the oxide superconductor, and as for these aluminum materials, there is a tendency that they are remained without being removed and oxidized with the oxygen contained in the oxide superconductor to cause the precipitation of aluminum oxide materials as impurities at the surface of the oxide superconductor, wherein there is a tendency for the aluminum oxide materials thus precipitated to react with the oxide superconductor to make the oxide superconductor to have a reduced critical current density. Hence, according to the manner described in document 1, although the occurrence of gaps among the crystal grains of the oxide superconductor could be prevented due to the aluminum materials embedded in the recesses or grain boundaries present at the surface of the oxide superconductor, but a problem is liable to entail in that the resulting wire composed of the oxide superconductor becomes to be insufficient in critical current density. Now, document 1 does not suggest anything about the formation of a stabilizing material which is essential in the case of a superconducting magnet.
According to the manner described in document 2, it is understood that the contact between the oxide superconductor and the silver tube due to the presence of the silver powder between them could be improved. But document 2 does not suggest anything not only about the prevention of the occurrence of gaps among the crystal grains of the oxide superconductor but also about the improvement of the resulting superconducting wire in terms of critical current density.
As for the manner described in document 3, there is a problem in that the production of a superconducting wire cannot be conducted at a desirably high production speed because such long period of time as above described is required in the heat treatment. In addition, there is also a problem in that since the formation of the superconducting material is conducted by the thin film-forming process, it is necessary to precisely control the composition of elements constituting the superconducting material upon the film formation wherein a slight variation in said composition upon the film formation makes the resulting superconducting material to greatly varied in superconducting characteristics, and because of this, it is almost impossible to produce a long superconducting wire having uniform superconducting characteristics.
As apparent from the above description, according to any of the conventional manners, there cannot be effectively realized a high quality superconducting wire having uniform superconducting characteristics and which is free of a reduction in the critical temperature, always exhibits an excellent current density and excels in mechanical strength.
An object of the present invention is to solve the foregoing problems in the prior art of producing a superconducting wire and to make it possible to effectively produce a high quality superconducting wire with no reduction in critical temperature and which always exhibits superconductor having a high critical temperature.
Another object of the present invention is to provide a superconducting wire having uniform superconducting characteristics and which is free of a reduction in the critical temperature, always exhibits an excellent critical current density and excels in mechanical strength, said superconducting wire having a stacked structure comprising (a) an electrically conductive substrate, (b) a layer composed of an oxide superconductor having a high critical temperature and (c) a layer composed of an electrically conductive material which is substantially not reactive with the oxide superconductor of said layer (b), wherein the electrically conductive material of said layer (c) is impregnated in said layer (b) such that the electrically conductive material of the layer (c) fills gaps among the crystal grains in the layer (b) while reaching the interface between the substrate (a) and the layer (b).
A further object of the present invention is to provide a superconducting wire having uniform characteristics and which is free of a reduction in the critical temperature, always exhibits an excellent critical current density and excels in mechanical strength, said superconducting wire having a stacked structure comprising (b) a layer composed of an oxide superconductor having a high critical temperature interposed between (a) a tape-like shaped electrically conductive substrate and (c) a tape-like shaped layer composed of an electrically conductive material which is substantially not reactive with the oxide superconductor of said layer (b), wherein the electrically conductive material of said layer (c) is impregnated in said layer (b) such that the electrically conductive material of the layer (c) fills gaps among the crystal grains in the layer (b) while reaching the interface between the substrate (a) and the layer (b).
A further object of the present invention is to provide a superconducting wire having uniform superconducting characteristics and which is free of a reduction in the critical temperature, always exhibits an excellent critical current density and excels in mechanical strength, said superconducting wire having a stacked structure comprising (a) a tublar electrically conductive substrate, (c) a layer composed of an electrically conductive material disposed to cover the inner wall face of said tublar electrically conductive substrate such that said tublar electrically conductive substrate has a core space, and (b) a layer composed of an oxide superconductor having a high critical temperature disposed to fill said core space of said tublar electrically conductive substrate, said electrically conductive material of said layer (c) being substantially not reactive with said oxide superconductor of said layer (c), wherein the electrically conductive material of said layer (c) is impregnated in said layer (b) such that the electrically conductive material of the layer (c) fills gaps among the crystal grains in the layer (b) while reaching the interface between the substrate (a) and the layer (b).
A further object of the present invention is to provide a process for producing a superconducting wire having uniform superconducting characteristics and which is free of a reduction in the critical temperature, always exhibits an excellent critical current density and excels in mechanical strength, said process comprising the steps of: providing a stacked body comprising (a) an electrically conductive substrate, (b) a layer composed of an oxide superconductor having a high critical temperature and (c) a layer composed of an electrically conductive material which is substantially not reactive with said oxide superconductor of said layer (b); subjecting said stacked body to heat treatment to fuse said layer (c) to impregnate in said layer (b) such that the electrically conductive material of the layer (c) fills gaps among the crystal grains in the layer (b) while reaching the interface between the substrate (a) and the layer (b).